1. Field of the Invention
The present invention relates to a gas sensor element and a gas sensor suitably employed for determining the concentration of a particular gas contained in, for example, combustion gas or exhaust gas of a combustor, an internal combustion engine, etc.
2. Description of the Related Art
Conventionally, a gas sensor is used for determining the concentration of a particular component (oxygen, etc.) in the exhaust gas of an internal combustion engine. The gas sensor has a gas sensor element provided therein. The gas sensor element has, on a forward end side thereof, a detection element including a solid electrolyte body and a pair of electrodes disposed on the solid electrolyte body. The gas sensor element may be exposed to poisoning substances (e.g., silicon and phosphorus) contained in exhaust gas and may suffer adhesion of water droplets contained in exhaust gas. Thus, in order to trap poisoning substances and to prevent direct contact of water droplets with the gas sensor element, the outer surface of the gas sensor element is covered with a porous protection layer. That is, the detection element, which is exposed to a gas-to-be-measured (exhaust gas), is entirely covered with the porous protection layer.
Also, the following technique has been developed: the porous protection layer is formed of two layers (i.e., an upper layer and a lower layer), and, by means of the lower layer having a porosity higher than that of the upper layer, an anchor effect brings the upper layer into close contact with the lower layer, whose surface is roughened by pores (see Patent Documents 1 and 2).    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2003-322632 (claim 15)    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2007-206082 (claim 15)
3. Problems to be Solved by the Invention
By employing the porous protection layer having a two-layer structure such that the lower layer has a higher porosity than the upper layer, the total volume of voids (empty spaces) contained in the lower layer increases, thereby imparting a thermal insulation property to the lower layer. Therefore, even when the upper layer is cooled by adhesion of water, the gas sensor element on the inner side is unlikely to be suddenly cooled. Even in a state where the detection element is heated by means of a heater, damage to the gas sensor element caused by adhesion of water can be effectively suppressed.
However, when the porosity of the porous layer is increased, a problem may arise in that the number of bonding sites between particles forming the porous layer is reduced, resulting in a layer of decreased strength. Meanwhile, a method has been proposed in which a porous layer is formed from particles having a smaller particle size (hereinafter the particles are also referred to as “fine particles”) for the purpose of increasing the strength of the porous layer while maintaining the porosity of the porous layer at a high level. When the porous layer is formed from a slurry containing fine particles, the strength of the layer is increased, because of an increase in the number of bonding sites between particles. However, the amount of solvent is reduced while drying the slurry, and thus cracking is likely to occur in the layer due to separation of bonded fine particles by surface tension.